1989 merc black max xr4 prop questions

[gschultz05]

I have a 1989 cheetah limited surface tunnel hull with a mercury 150 black max xr4, the boat gets out of the hole fine, but dont want to go much more than 5k rpm and only around 45-50 mph if that. Current prop is a 3 blade 14 1/4 x 22. trying to figure out if i have a prop problem or a mechanical issue

 

[racerone]

Start with a compression test.----Post your numbers here.

 

[gschultz05]

Start with a compression test.----Post your numbers here

First thing I did when I got the boat about 2 weeks ago, numbers were 140 across the board using otc compression gauge

 

[Chris1956]

Are your trimming it until she porpoises? Does the nose rise?

Did you check the hull for straightness? A hook in the hull will cause the bow to dive and drag.

How long and heavy is the hull? What is max HP rating?

 

[gschultz05]

Are your trimming it until she porpoises? Does the nose rise?

Did you check the hull for straightness? A hook in the hull will cause the bow to dive and drag.

How long and heavy is the hull? What is max HP rating?

The boat rides great in the water, doesn't porpoise, trims is good, isn't plowing at all talked to some of the shops here and all agreed that the 22pitch prop is to much for this 150 and that I should go to around a 19 pitch

 

[QBhoy]

Hi. Ok a normal 150 2.0 I think you’d be not far off or a little under pitched…but do those engines you have not have the smaller gear box with the higher ratio than normal ? Think the mariner magnum version or maybe the merc xr other models do anyway.

 

[gschultz05]

Hi. Ok a normal 150 2.0 I think you’d be not far off or a little under pitched…but do those engines you have not have the smaller gear box with the higher ratio than normal ? Think the mariner magnum version or maybe the merc xr other models do anyway.

It is an 150 black max xr4

 

[QBhoy]

It is an 150 black max xr4

Yeah. But do some of the xr models not have the smaller gearcase and higher ratio than normal ?

 

[QBhoy]

It is an 150 black max xr4

So just looked it up. I was right. You have a higher ratio than normal. It’s a 1.78 to one or something like that. Most 150 2 stroke around then were 1.9-1, 2-1 or close.
That explains everything really. I think it’s also the smaller gear case too.

 

[Chris1956]

For max speed, they boat needs to trim up just before it porpoises. If you cannot get it to that point, some speed is lost.

A prop for max speed usually achieves that speed and RPM, when trimmed to max. If does not sound like you have this scenario. Engine could be too small, prop pitch could be too great, hull could not be sufficiently trimmable.

What is max HP for that hull?

 

[Texasmark]

I'm assuming a 1.87 since you are into 14+" props and not 13+" props which would be the smaller 4" dia. housing, 1.76 gear ratio on your engine to prop. With that said, per (https://docs.google.com/document/d/1axsbSmCOZZBLIE6TUahFbAIbenrB3fn3ehb6G698FmE/edit?usp=sharing) your slip is 11% which isn't all that bad considering other boat, hull conditions, mounting, and setup factors, without regard to engine problems.

The tunnel hull isn't your best planing surface....a flat board is the best. I think your engine tops out at 5600 RPM for max rating and your 5k is at the bottom of the range where you are near "lugging" the engine which is not good for it. Dropping to a 19 should give you around 600 RPM and provide the following results...more or less:
https://docs.google.com/document/d/1axsbSmCOZZBLIE6TUahFbAIbenrB3fn3ehb6G698FmE/edit?usp=sharing.

A 20 would be better if you find one with similar characteristics to your current prop. On the 7% slip number, I doubt that would be obtainable with a tunnel but changing props is always a crap shoot.

 

[QBhoy]

I'm assuming a 1.87 since you are into 14+" props and not 13+" props which would be the smaller 4" dia. housing, 1.76 gear ratio on your engine to prop. With that said, per (https://docs.google.com/document/d/1axsbSmCOZZBLIE6TUahFbAIbenrB3fn3ehb6G698FmE/edit?usp=sharing) your slip is 11% which isn't all that bad considering other boat, hull conditions, mounting, and setup factors, without regard to engine problems.

The tunnel hull isn't your best planing surface....a flat board is the best. I think your engine tops out at 5600 RPM for max rating and your 5k is at the bottom of the range where you are near "lugging" the engine which is not good for it. Dropping to a 19 should give you around 600 RPM and provide the following results...more or less:
https://docs.google.com/document/d/1axsbSmCOZZBLIE6TUahFbAIbenrB3fn3ehb6G698FmE/edit?usp=sharing.

A 20 would be better if you find one with similar characteristics to your current prop. On the 7% slip number, I doubt that would be obtainable with a tunnel but changing props is always a crap shoot.

Think it’s a 1.78. Could be wrong.

 

[QBhoy]

I have a 1989 cheetah limited surface tunnel hull with a mercury 150 black max xr4, the boat gets out of the hole fine, but dont want to go much more than 5k rpm and only around 45-50 mph if that. Current prop is a 3 blade 14 1/4 x 22. trying to figure out if i have a prop problem or a mechanical issue

What 3 blade prop is that, at those dimensions? Sounds an odd ball perhaps

 

[Texasmark]

Think it’s a 1.78. Could be wrong.

The sales brochure lists both. In second guessing the model and what its designed to do, the smaller case is designed to go fast so one would expect, with the smaller 13" wheel, the prop would turn faster so that you were going faster at WOT RPM with a light boat and a lot of pitch in the prop, whereas the larger case and more of a reduction gear, larger prop would be to push heavy loads.... large, heavy, Bass Boats with a lot of "stuff" at the stern and poor stern lift with the high deadrise to support running on the Pad.......buttttt as you say, my second guessing could be wrong also.

Edit: That along with upping the displacement from 2L to 2.5L on some of these engines to get more torque to spin the larger prop under a heavier loaded boat.